Packing boxes are used in many commercial, industrial and consumer applications to provide a conformable sealing member around a movable shaft or stem inserted through a cylindrically shaped cavity of larger diameter, generally called a stuffing box. In compressor applications, a compressive load is typically created by pressure (that is intended to be contained) and produces a combination of a radial force that forces the inner diameter of the packing to conform to the outer diameter of the movable shaft or stem, and an axial force that forces the face of the packing facing away from the pressure against one of the interior faces of the packing box or packing cups, so as to form a seal between the packing, packing box, and the movable shaft.
Since the function of the packing material is to form a pressure seal around a movable rod, an undesired result of the contact between the movable rod and the packing material is friction, which produces heat. The greater the pressure being sealed by the packing, the greater the friction and the resulting heat production becomes. In extreme cases, the heat produced can melt or damage the packing.
Common causes of packing failure in equipment such as valve packing boxes, pumps, compressors, probes, and the like may include improper loading, over or under tightening of packing cups, and misalignment of the shaft or stem. Too much compression may lead to excessive friction and premature wear, while too little compression can lead to premature leakage. Shaft or stem misalignment can generate these problems as well. Packing integrity can change by over-pressuring, surging, flashing, excessive temperature, and combinations thereof by the material flowing through the valve packing box. Such occurrences may cause over-compression of the packing inside the stuffing box, thereby creating a leakage path even when springs are used to maintain compression. Changes in packing compression may also cause control valve packing boxes to stick and slip, resulting in unbalancing process control. Pumps, compressors, probes, mixers, and the like, can also suffer from initial improper packing integrity and changes over time, which may result in unwanted emissions, safety problems, maintenance downtime, and loss of production.
Many packing boxes that are designed to work in extreme pressure environments use various liquid coolants flowed through cooling jackets external to the packing cups for cooling the exterior of the packing cups. However, significant cooling is not provided to the packing material within the packing cups since, although these jackets permit significant coolant flow, the considerable distance from the packing in the box to the coolant does not allow efficient heat transfer from the packing itself; thus, while the outside of the boxes are well cooled, the packing material itself may not be. This uneven cooling or lack of cooling can cause the packing material within the packing cups to degrade and melt.
Additionally, cooling methods in commercial packing boxes utilize tangential coolant channels for permitting coolant to circulate through a packing cup prior to passing into the next packing cup. Since the packing cups are cooled serially, the coolant must travel through a lengthy path which increases the pressure drop in the coolant across the packing box, thereby reducing the flow rate of coolant and its resulting cooling efficiency. That is, the small amount of coolant that travels through the packing box may become heated in the first few packing cups to the point that it does not provide cooling to subsequent cups. High-pressure coolant pumps may be used to force the coolant through the packing box, which may be undesirable in some situations due to cost, weight, size, and increased parts and maintenance requirements of such pumps.
Degradation and melting of the packing material within the packing cups can lead to overheating of the packing box due to the high level of friction and heat that is generated by the rod movement. This overheating can cause significant damage not only to the packing box, but to the rod and piston assembly of a compressor including binding of the rod or the piston assembly. Similar damage may occur for pumps using a rotating impeller, as an example. Resulting mechanical problems within the compressor unit, and the potential for bodily injury to individuals working in the area around the compressor unit are of concern.